Contribution of proton and electron precipitation to the observed electron concentration in October–November 2003 and September 2005

P T Verronen,M Palmroth,E Armandillo,K Kauristie,T E Sarris,C.-F Enell,E R Talaat,M E Andersson,A Kero,J M Wissing

doi:10.5194/angeo-33-381-2015

Abstract

Abstract. Understanding the altitude distribution of particle precipitation forcing is vital for the assessment of its atmospheric and climate impacts. However, the proportion of electron and proton forcing around the mesopause region during solar proton events is not always clear due to uncertainties in satellite-based flux observations. Here we use electron concentration observations of the European Incoherent Scatter Scientific Association (EISCAT) incoherent scatter radars located at Tromsø (69.58° N, 19.23° E) to investigate the contribution of proton and electron precipitation to the changes taking place during two solar proton events. The EISCAT measurements are compared to the results from the Sodankylä Ion and Neutral Chemistry Model (SIC). The proton ionization rates are calculated by two different methods – a simple energy deposition calculation and the Atmospheric Ionization Model Osnabrück (AIMOS v1.2), the latter providing also the electron ionization rates. Our results show that in general the combination of AIMOS and SIC is able to reproduce the observed electron concentration within ± 50% when both electron and proton forcing is included. Electron contribution is dominant above 90 km, and can contribute significantly also in the upper mesosphere especially during low or moderate proton forcing. In the case of strong proton forcing, the AIMOS electron ionization rates seem to suffer from proton contamination of satellite-based flux data. This leads to overestimation of modelled electron concentrations by up to 90% between 75–90 km and up to 100–150% at 70–75 km. Above 90 km, the model bias varies significantly between the events. Although we cannot completely rule out EISCAT data issues, the difference is most likely a result of the spatio-temporal fine structure of electron precipitation during individual events that cannot be fully captured by sparse in situ flux (point) measurements, nor by the statistical AIMOS model which is based upon these observations.

Highlights

Solar energetic particle precipitation affects the neutral composition of the upper stratosphere, mesosphere and lower thermosphere in the polar regions (Sinnhuber et al, 2012; Verronen and Lehmann, 2013)
For Case 1, electron and proton effects peak at about 110 and 60 km indicating strongest forcing by 6 keV and 20 MeV particle energies, respectively
Comparing the proton forcing from CSDA and Atmospheric Ionization Module Osnabrück (AIMOS), the profiles show similar altitude behaviour below 90 km, the AIMOS values are consistently about a factor of 2 larger

Summary

Introduction

Solar energetic particle precipitation affects the neutral composition of the upper stratosphere, mesosphere and lower thermosphere in the polar regions (Sinnhuber et al, 2012; Verronen and Lehmann, 2013). Verronen et al.: Contribution of electron and proton precipitation precipitation may modulate regional ground-level climate on solar cycle timescales (Rozanov et al, 2005; Seppälä et al, 2009; Baumgaertner et al, 2011). Most of the precipitating particles deposit their energy in the mesosphere and lower thermosphere, and the connection to stratospheric ozone requires descent of produced odd nitrogen inside the wintertime polar vortex (Callis and Lambeth, 1998; Siskind et al, 2000; Randall et al, 2009; Päivärinta et al, 2013). The altitude of initial odd nitrogen production is important for the efficiency of the odd nitrogen descent, especially around the mesopause where the wintertime mean circulation changes vertical direction (Smith et al, 2011)

Results

Discussion

Conclusion